A well-known way of mounting turbine blades around the periphery of a turbine rotor, as described in German Patent application DE 10 2008 031 780 A1, comprises the so-called “pinned root fixing”, in which radially and circumferentially extending flanges, called “disc fingers”, on the periphery of the turbine rotor disc and corresponding “blade fingers” on the turbine blade root are inter-digitated with each other and fixed together by means of cylindrical metal rods, known as “pins”, which pass axially through the blade fingers and the disc fingers. Such arrangements are particularly known for use on impulse blading in wet steam conditions. An example of such a blade is illustrated in FIGS. 1 and 2. FIG. 1 is a three-dimensional perspective view on the pressure side of a rotor blade unit 10 and FIG. 2 is a radial section through the periphery of a turbine rotor disc 20, showing how the disc is adapted for attachment of the turbine blade of FIG. 1.
Referring first to FIG. 1, when the blade unit 10 is oriented for operation in the turbine, its aerofoil 11 extends between a radially outer shroud 12 and a radially inner platform 13. Extending radially inwardly from the platform 13 is a blade root 14, which is divided into a number (in this particular case, four) of identical blade fingers 141, the fingers being of length L, axially spaced apart from each other, and mutually parallel. Each blade finger 141 has a radially outer portion 142 of breadth “b1” and a radially inner portion 143 of breath “b2”, where b1>b2, and the transition between the inner and outer portions is marked by shoulders 144 Each outer portion 142 of the blade fingers has a through bore 145 of diameter “D1”, and each inner blade finger portion 143 has a through bore 146 of diameter “D2”. The bores 145 in the outer blade finger portions 142 are identically dimensioned and arranged axially in-line with each other. Similarly, the bores 146 in the inner blade finger portions 143 are identically dimensioned and arranged axially in-line with each other. In general, D1=D2.
Turning to FIG. 2, the periphery of the rotor disc 20 is divided into a number of radially and circumferentially extending, mutually parallel disc fingers 201, which are axially spaced apart from each other by radially and circumferentially extending identical grooves 202. The blade fingers are accommodated in the grooves 202 between the disc fingers; hence the number of disc fingers (in this particular case, five) is one more than the number of blade fingers. The grooves are of radial depth L2, which is the same as L1 except for a relief at the bottom of the grooves to prevent contact with the ends of the blade fingers 141. The grooves are dimensioned with appropriate tolerances to accept the fingers 141 of the blade root 14 as a sliding clearance fit. Hence, the radially inner finger portions 143 of the blade root 14 fit into radially inner portions 203 of grooves 202, of nominal breadth b2, and the radially outer finger portions 142 of the blade root 14 fit into radially outer portions 204 of grooves 202 having a nominal breadth b1. Consequently, the disc fingers 201 are shaped in a way that is complementary to the blade fingers 141, in that they have radially inner portions 205 of increased width relative to their radially outer portions 206, the transition between the inner and outer portions being marked by shoulders 207. Generally, the breadth b1 of the blade fingers is nominally the same as the breadth b1 of the disc fingers. Each inner disc finger portion 205 has a through bore 208 of diameter “D2”, and each outer disc finger portion 206 has a through bore 209 of diameter “D1”. Bores 208 and 209 match the bores 146 and 145 in the inner and outer blade finger portions 143 and 142, respectively. The radial dimensions of the disc fingers 201 and the blade fingers 141 are closely matched, so that when the blade fingers 141 are inserted into the grooves 202, shoulders 144 on the blade fingers 141 butt up against shoulders 207 on the disc fingers 201, bores 146 are axially in-line with bores 208, and bores 145 are axially in-line with bores 209. Appropriately dimensioned cylindrical pins (not shown) can therefore pass in a sliding clearance fit through the holes in the blade fingers 141 and the disc fingers 201 in order to attach the blades to the disc.
For economic and manufacturing reasons, the disc is made from a low alloy steel, comprising about 1 wt. % to about 3 wt. % nickel, whereas it is necessary to make the blades from a high alloy steel, comprising for example about 12 wt. % chromium, in order to ensure they have adequate resistance to water droplet erosion and high steam temperatures. It is well known that the area of the root sustaining the moving blades can be prone to SCC, which is caused by high peak stresses induced by contacts between root portions under high centrifugal loads when the steam turbine rotor is operating with steam close to saturation. The problem is further increased when the steam contains impurities that accelerate corrosion.